Non-shivering thermogenesis in brown adipose tissue (BAT) has evolved as a means of extra heat production in neo-natal mammals (including man) and during cold adaptation and hybernation. The mechanism involves a reversible alteration in the passice proton conduction of BAT mitochondrial membranes with a subsequent release of fatty acid oxidation from the restraints of respiratory control. Physiological regulation of thermogenesis at the cellular level involves interaction of purine nucleotides and probably acyl CoA with a 32,000 Mr protein component which may be unique to BAT mitochondrial membranes. While some details of the mechanism are established, the actual pathway for proton conduction across the BAT mitochondrial membrane remains a mystery. The fact tha BAT mitochondria are highly permeable to both H+ and anions Cl-, No3-) suggests that the nucleotide binding 32,000 Mr protein may form a non-specific pore through which diffusion occurs. However, the specific proton conductance of BAT mitochondria suggests that the 32,000 Mr protein forms a channel which is similar to the DCCD-sensitive channel in the FO portion of H+ ATPase. The proposed experiments are designed to destinguish between these possibilities by isolating from BAT mitochondria the component which are required to reconstitute a nucleotide-sensitive proton conduction pathway in liposomes. If the 32,000 Mr protein will reconstitute this activity then subsequent studies using chemical reagents and monoclonal antibodies will determine the orientation of this protein in the membrane as well as identify the functional peptides. The medical significance of this research is related to reports of deficient diet-induced thermogenesis in animals which are genetically predisposed to obesity. In addition, hypothermia in humans, especially among the elderly, has been attributed to inadequate non-shivering thermogenesis. Finally, the general phenomena of uncoupling in mitochondria may be related to the more highly developed mechanism observed in brown adipose tissue mitochondria